1. Field of the Invention
This invention relates generally to capacitors, and, more particularly, to cathode materials used in a capacitor of an implantable medical device.
2. Description of the Related Art
Since their earliest inception some forty years ago, there has been a significant advancement in body-implantable electronic medical devices. Today, these implantable devices include therapeutic and diagnostic devices, such as pacemakers, cardioverters, defibrillators, neural stimulators, drug administering devices, among others for alleviating the adverse effects of various health ailments. Today's implantable medical devices are also vastly more sophisticated and complex than their predecessors, and are therefore capable of performing considerably more complex tasks for reducing the effects of these health ailments.
Usually, body-implantable medical devices rely on battery power to perform their therapeutic and/or diagnostic tasks. The battery supplies power to the electrical components of the implantable medical device, and also typically provides power to a capacitor of the defibrillator device, which stores the energy supplied thereto from the battery. The capacitor typically includes an anode, a cathode, and a fluid electrolyte disposed between the anode and cathode. The various types of materials used for the anode, cathode, and electrolyte may have an impact on the capacitor's ability to store energy from the battery, and the rate at which the energy is stored prior to discharging the capacitor. Importantly, the materials may also affect the volume of the device. Smaller defibrillator devices, which in turn require smaller capacitors, will typically enhance the patient's comfort. Therefore, materials choices, which allow for designing a lower volume capacitor, are of specific interest.
Typically, in an implantable medical device, the capacitor is used to deliver therapeutic electric signals to the patient's heart in response to the device receiving abnormal feedback signals from the heart. The therapeutic electric signals delivered to the patient's heart may vary somewhat in intensity depending on the patients' physiology and the details of the implant. Typically, the electric pulse energy delivered to the heart is of the order of 30 J for a single defibrillation pulse. The energy stored in the capacitor has to be somewhat larger due to losses along the delivery path during the release of the energy.
The capacitor, therefore, plays a vital role in the implantable defibrillator device for if the energy supplied from the battery is not stored in a timely manner within the capacitor prior to its discharge or if the energy is not released in a timely manner during its discharge, the capacitor may not be able to deliver sufficient energy to the patient's heart at a critical point in time when deemed necessary by the implantable medical device. As a result, the patient's health may be adversely affected by the capacitor's inability to adequately and/or quickly store the energy supplied by the battery. Therefore, using electrical currents of the order of 10 mA, capacitor charge times are typically of the order of 10 seconds. Discharge times are typically of the order of 10 milliseconds. In summary, the capacitor bank of an implantable defibrillator will have to be able to deliver about 30 J of electrical energy in a total time window of about 10 seconds, using a charge current of the order of 10 mA. For the design engineer, a low charge and discharge time directly translates into a low internal resistance, or more generally speaking, impedance, of the capacitor bank in the defibrillator device. The impedance behavior of a capacitor is technically characterized by its' equivalent series resistance (ESR) value measured at a specified frequency. For a capacitor bank in a defibrillator device, the ESR measured at 120 Hz is typically of the order of 5 Ohms or less, in order to accomplish timely delivery of the therapeutic electrical pulse with minimal waste of energy lost in heating the device. The technical solution is generally sought in a capacitor design in which the anode of the individual capacitor within a bank is charged to positive potentials between 150 and 400 Volts, with charge storage capabilities ranging from about 200 to about 500 micro-Farad. As is well known to those skilled in the art, a high potential Va on the anode together with a fairly low capacitance Ca has to be approximately balanced with a low potential Vc and a high capacitance Cc on the cathode side:VaCa≈VcCc 
The potential on the cathode is limited to about 1.2 V, which is the potential at which electrolysis occurs in water based electrolytes at temperatures around room temperature. Electrolysis in turn causes gas formation and therefore should be avoided. Therefore, the cathode capacitance needs to be approximately 80 mF. Given the additional requirement of a small overall volume for the capacitor, this means that specific capacitances of the order of 20 mF/cm2 or above are needed. Therefore, a cathode is desirable which has both, a high specific capacitance and, at the same time, a low ESR in the defibrillator capacitor application. The present invention is directed towards implementing new ways to produce low ESR, high capacitance cathode electrodes in an economical fashion, utilizing both, novel materials and materials already known to those skilled in the art.
Previous authors have addressed various aspects of this invention: in 1988, Libby (U.S. Pat. No. 4,780,797, later also in U.S. Pat. No. 4,942,500 and U.S. Pat. No. 5,043,849) purportedly discloses a novel capacitor concept by combining a porous Ta anode with a cathode made from an alloy of Ta and a member of the Pt metal family. With the regard to the cathode used in this capacitor, this concept was very likely based in part upon the early scientific results published by Trasatti, Trasatti et al., and also from Raistrick. These authors appear to have found that oxides from the group of metals consisting of hafnium, palladium, iridium, ruthenium, molybdenum and others exhibited a phenomenon, for which the term “pseudo-capacitance” was later coined (4). Materials exhibiting pseudo-capacitance can store amounts of electrical charge, which may exceed the value expected from pure geometrical considerations by orders of magnitude. Therefore, the authors argue that these materials are well suited as cathodes in electrochemical capacitors with liquid electrolytes. The actual capacitance achievable with the specific cathode suggested by Libby, however, appears to be comparatively low. In a defibrillator application, this would mean that an impractically large number of individual capacitors would have to be connected in series in order to achieve the goal of providing approximately 30 J to defibrillate the heart—therefore, the inventors contend that Libby's approach would likely not be suitable for this application.
A similar capacitor concept was later disclosed by Loeb in 1994 (U.S. Pat. No. 5,312,439). In this reference a porous electrode preferably constructed from Ta is suggested to be balanced with a high capacitance cathode constructed preferably from “activated” iridium. In the context of this particular reference “activated iridium,” means a thin, chemisorbed layer of iridium oxide generated by heating bulk iridium metal in air using an acetylene torch. Specifically included in the cathode material list of this reference were oxides of the metals hafnium, palladium platinum and others. The inventors note that ruthenium oxide was apparently not disclosed, perhaps because Loeb suggested an in vivo application in which the electrode materials would have contact with body fluids and ruthenium oxide is generally known to be toxic. The cathode design, iridium with an iridium-oxide layer chemisorbed on the metal, seems to provide some desirable properties for the application; that is, both a high capacitance and a low ESR. However, iridium is extremely expensive. Therefore, the specific form of the iridium-oxide based cathode electrode with solid iridium metal as the substrate is generally acknowledged as not economical for a defibrillator capacitor application, where capacitances in excess of about 20 mF/cm2 are needed.
Shortly after the Loeb patent issued, a number of U.S. patents issued to Evans relating to a capacitor using a high potential, low capacitance porous anode which as understood by the inventors were constructed preferably of Ta balanced with a low potential, high capacitance cathode, both in contact with a liquid electrolyte. The cathode material, chosen from a group of metal oxides including the oxides of platinum, iridium, hafnium, palladium and ruthenium—with ruthenium oxide being the preferred material—would be spray-coated onto the inside of the metallic case of the liquid electrolytic capacitor. Clearly, the suggested coating process does not allow for the formation of a chemisorbed high capacitance layer; rather, the high capacitance material is simply adsorbing on the surface of the substrate. Therefore, the resulting mechanical bond strength and the electrical conductivity between the substrate and adsorbate are lower than may be desirable in demanding applications. A study of the relevant patent literature will reveal the following patents issued to Evans: U.S. Pat. Nos. 5,369,547; 5,469,32; 5,559,667; 5,737,181; 5,737,181, 5,754,394 and 5,982,609.
A modification of the cathode construction discussed above, namely that of the capacitance per unit area or specific capacitance was suggested later in a patent by Hudis (U.S. Pat. No. 6,208,502). Hudis purportedly detailed the use of a sol-gel derived form of ruthenium oxide for a cathode application in a high voltage capacitor application. This form of ruthenium oxide appears to have a specific capacitance even higher than the previously known forms of ruthenium oxide. However, the coating method suggested by Hudis—namely screen-printing the cathode material onto a substrate metal surface—does not appear to lead to the formation of a chemisorption bond between the cathode substrate metal and the high capacitance layer. Therefore, the inventors believe that the ESR of such capacitors using this type of cathode is likely not as low as it is desirable in the defibrillator capacitor application.
All of the prior art patents referenced so far neither discuss nor claim a multi-layered structure of the low potential, high capacitance cathode surface in order to obtain a low ESR value of the capacitor. However, a related structure is discussed in the German “Patentoffenlegungsschrift DE 198 36 651 A1”. This publication purportedly describes a sandwich-type structure having layers of material (e.g., metal-metal-carbide-carbon) for use in low-potential supercapacitor applications. The structure apparently may be generated by spray-painting a metal surface with graphite and heating the surface subsequently in vacuum and in air. Metal carbides do have conductivities comparable to those of metals and therefore this layered structure will result in a very economical capacitor with low ESR and moderately high specific capacitance. However, the capacitance values needed for the high voltage defibrillator application are higher than those achievable with construction discussed this particular reference.